German Pat. No. 1,778,559 (U.S. Pat. No. 3,633,830) describes a process for the comminution of materials which are relatively soft, stretchable, plastic or thermoplastic at normal temperatures and hence tend to be stretched upon milling at such temperatures. Such materials include thermoplastic foil scraps and the like. In this process the material to be comminuted is subjected to embrittlement by direct heat exchange with a cold gas stream and, after embrittlement, is conveyed through the mill by a carrier gas stream.
The cooling gas stream and the conveyor gas stream can be the same gas, e.g. gas derived from liquified nitrogen, the latter being mixed with one or both of the gas streams or with the common gas stream, the excess being discharged from the apparatus after heat exchange with the comminuted material.
Especially in the case of synthetic-resin scraps, the material to be comminuted and the comminuted material can contain easily volatilized substances, e.g. plasticizers, which cannot be safely released into the atmosphere even in the smallest quantities. The conventional process thus has significant disadvantages. For example, if release of such easily volatilized substances is to be avoided and a portion of the gas serving as the carrier or cooling gas is released into the atmosphere, it is necessary to provide additional devices, such as filters, adsorbers or the like, to recover the easily volatilized substances. In practice it is found that the latter substances are only partly removed by such techniques.
Another disadvantage of the conventional process, especially when liquefied nitrogen is used as the cooling source, is that the nitrogen carrier gas present in the mill has a relatively high density and, because the mill operates at high speed, considerable energy loss is encountered as a result of gas friction.